Expiration of approval. Unless construction or use is commenced and diligently pursued within one calendar year from the date of granting of the variance, such variance shall not become vested and shall be considered null and void without further hearing or action by the Zoning Board of Appeals. Upon specific application, in writing, by the owner of the affected property prior to the date of such expiration, the Zoning Board of Appeals may, however, extend, in its discretion, the expiration date for a maximum of two periods, not to exceed six calendar months each, provided that the circumstances and facts relied upon in the granting of the variance have not materially changed.